The present invention is directed to novel high performance filters having characteristics suitable for use in various filtration applications, such as microfiltration. More particularly, this invention is directed to a novel high performance filter media system comprising a fiber reinforced matrix composite that is low in cost, durable, resistant to chemicals and high temperatures, not subject to particulation, high in mechanical strength and separation efficiency, and biocompatible.
The prior art provides many types of materials which remove, filter, or capture gases and particulate materials. These filters of the art, while fairly effective in the applications for which they were designed, do not offer the efficiency, performance, and durability demanded by new, high performance applications.
The demand for higher quality materials, reduced manufacturing costs, and environmentally clean processes is forcing industry to move away from traditional methods of separation and purification, such as distillation and pasteurization, towards the use of filtration. Filter systems are now capable of offering low energy, more efficient, and environmentally friendly operations. Unfortunately, the widespread use of high performance filtration is restricted by the lack of suitable filter media materials. Such media must offer low cost; durability; chemical resistance, particularly to acids and alkalis; resistance to high temperatures, for both operation and sterilization purposes; no particulation (i. e., release of filter media particles into the filtrate stream); mechanical strength to cope with pressure swings; separation efficiency, particularly for particles in the 0.1 to 100 microns range; and biocompatibility for certain applications such as the filtration of blood.
Table 1 below lists the currently available filter media materials together with their advantages and disadvantages.
As presented in Table 1, no one filtration material offers the required balance of properties needed for new, high performance applications.
Ceramic filter media have made some inroads, however their acceptance is hampered by the following: high cost because expensive and complex manufacturing processes are required; susceptibility to attack by alkalis; limited durability because of their inherent brittleness; and difficulties in controlling pore size distribution and permeability, which are critical aspects of high performance filter media.
High temperature composite materials, in which a ceramic or carbon matrix is reinforced with a continuous fiber, are used in a variety of applications. They are most commonly used in aircraft brakes. In this application, the braking material is made from a carbon matrix reinforced with carbon fibers (carbon/carbon or C/C). Such materials have a high mechanical strength and are capable of operating at extreme temperatures, up to 3000xc2x0 C. in a non oxidizing atmosphere. Composites in which both the reinforcing fiber and the matrix are both ceramic are used in specialty applications. In particular, they are used in aircraft engine parts where strength at high temperatures and low weight are needed.
Such high temperature composite materials do offer some potential for use as filter media. For example, carbon/carbon composites, due to the excellent balance of properties, have found use as a filter support. U.S. Pat. No. 4,944,996 discloses the use of a carbon/carbon support intended to receive a mineral membrane for separation procedures. U.S. Pat. No. 4,500,328 discloses the use of carbon/carbon composites to filter radioactive waste, and the use of activated carbon fiber to increase surface area. U.S. Pat. No. 5,183,546 discloses an electrochemical filter consisting of an electrically conductive fibrous material that contains microscopic particles of carbon or active charcoal.
Ceramic matrix composites have been used as hot gas filters. U.S. Pat. No. 4,968,467 discloses the use of refractory ceramic fibers matted together with a high temperature binder, such as colloidal alumina or silica, to form a tube like xe2x80x9ccandle filter.xe2x80x9d U.S. Pat. No. 5,196,120 discloses the use of a ceramic fiber-ceramic composite filter composed of ceramic fibers, preferably texturized, a carbonaceous layer thereover, and a silicon carbide coating over the carbonaceous layer, which coats substantially all of the fibers. A strong, light weight filter is achieved.
Despite the advances made in the art, of which the above are examples, ceramic and carbon based composite materials have not previously been suited to high performance filtration. This is especially true for microfiltration because of the difficulties in achieving the required porosity, surface area and permeability required for efficient separation. In general, pore size distribution and the ability of the filter to retain or capture particulate matter is a function of the fiber diameter (Filters and Filtration Handbook, Third Edition, 1992). In the art outlined above, fiber diameters range from 7 microns for conventional carbon fibers to 100 microns and above for some ceramic fibers. The diameters of such fibers are too large. These fibers do not provide the small pores required for efficient small particulate retention.
U.S. Pat. No. 5,138,546 discloses the addition of small carbon or charcoal particles which improves surface area and particle capture ability. However, this type of filter is not suitable for most high performance applications, particularly in the foodstuffs and chemical industries. These structures exhibit poor bonding of the particles to the substrate. In addition, there is a tendency for such constructions to particulate, in other words, release undesired particles into the filtrate stream. Also, the addition of such particles can only be performed on a random basis. There is little control with respect to uniformity and positioning.
In light of the disadvantages of the prior art, there is therefore presently a need to develop high performance filter media capable of operating in the microfiltration regime which offer: low cost; durability; chemical resistance, particularly to acids and alkalis; resistance to high temperatures, for both operation and sterilization purposes; no particulation (i.e., release of filter media particles into the filtrate stream); mechanical strength to cope with pressure swings; separation efficiency, particularly for particles in the 0.1 to 100 micron range; and biocompatibility for certain applications, such as the filtration of blood.
The present invention is directed to a novel high performance filter system having characteristics suitable for use in various filtration applications, such as microfiltration. The filter system consists of a carbon or ceramic composite substrate comprising a carbon or ceramic matrix reinforced with carbon or ceramic fibers and an array of carbon or ceramic fiber whiskers xe2x80x9cgrownxe2x80x9d on the surface of the carbon or ceramic composite substrate. Optionally, the ceramic fiber whiskers may be grown in the bulk of the substrate.
In another embodiment, the present invention also provides a filter media system comprising a fibrous substrate of at least one of carbon and ceramic fibers, wherein an array of carbon or ceramic fiber whiskers have been grown onto the surfaces of the fibers comprising the fibrous substrate.
It is therefore an object of the present invention to provide a filter system with a substrate having a high degree of mechanical integrity and stiffness, which is capable of resisting pressure changes with pulsed flows.
It is another object of the present invention to provide a substrate with an open structure that aids in providing a high level of permeability.
It is a another object of the present invention to provide a filter system with whiskers that are grown from the fiber surface and hence are intimately bonded so that particulation is avoided.
It is another object of the present invention to provide a filter system with whiskers that are grown in the bulk of the substrate.
It is another object of the present invention to provide a filter system with a whisker layer, which because of its small fiber whisker size, provides a pore size distribution sufficiently small to trap particles in the range of about 0.1 to about 100 microns.
It is a further object of the present invention to provide a filter system with a whisker layer, which in the microfiltration range, provides a pore size distribution sufficiently small to trap particles in the range of about 0.2 to about 2 microns.
It is a further object of the present invention to provide a filter system that offers substantial versatility in construction so that a variety of constructions can be produced in order to cope with different filter applications.
It is also an object of the present invention to provide a filter system that is amenable to different forms of filter media such as thin plates, open cylinders, spiral constructions, and corrugated constructions, so that whiskers may be grown where desired (i.e. on the outside or inside the substrate).
It is a further object of the present invention to grow the whiskers in a grid or strip pattern on the surface to promote turbulent mixing and therefore better separation characteristics.
It is another object of the present invention to provide a process for manufacturing the inventive filter system which has the ability to position the whiskers where they are needed.
The manufacturing process of the present invention enables the whiskers to be grown in a specific location. This is important in various filter configurations, and is a distinct advantage over the prior art.
The invention includes a filter media system comprising a carbon or ceramic composite substrate which contains a carbon or ceramic matrix reinforced with carbon or ceramic fibers. The composite has an array of carbon or ceramic fiber whiskers grown onto its surface, or within the bulk of the composite.
The invention also includes a process for manufacturing the filter media system wherein a carbon fiber is provided that has been woven into a fabric, and a carbon matrix is deposited by a chemical vapor infiltration (CVI) or a liquid pressure infiltration (LPI) process (the ceramic matrix is deposited onto ceramic fiber reinforcement by a CVI process, from an aqueous slurry, or by use of a pre-ceramic polymer) at temperatures of about 900 to about 1200xc2x0 C., to achieve a weight gain of about 10 to 200%. This composite is treated with a solution of a metal catalyst salt, preferably comprising nickel chloride or ferric chloride, and is then heated in hydrogen at elevated temperatures to reduce the metal salt to metal. In one embodiment carbon whiskers are then grown on the surface of the nickel coated composite by decomposition of methane gas at about 1000xc2x0 C., in one embodiment, for approximately two hours. The fiber whiskers are grown on the composite surface using metal catalyzed chemical vapor deposition (CVD).
In one embodiment, the present invention provides a process for manufacturing a filter media system comprising a carbon composite substrate which comprises a carbon matrix reinforced with carbon fibers, wherein an array of carbon fiber whiskers have been grown onto said substrate, comprising:
a) providing the carbon fiber reinforcement, preferably which has been woven into a fabric of selected weave style;
b) depositing the carbon matrix by one of chemical vapor infiltration (CVI) and liquid pressure infiltration (LPI) process at temperatures of about 900 to about 1200xc2x0 C. in order to achieve a weight gain of about 10 to about 200%;
c) treating the composite with a solution of metal catalyst salt;
d) heating the composite in hydrogen at elevated temperatures to reduce the metal salt to metal; and
e) initiating whisker growth on at least one of the surface and the bulk of the metal deposited composite by decomposition of low molecular weight hydrocarbon gas at elevated temperature, (preferably at about 1000xc2x0 C. for an effective period of time, in one embodiment being about two hours).
In another embodiment, the present invention provides a process for manufacturing a filter media system comprising carbon fibers, wherein an array of carbon fiber whiskers have been grown onto the surfaces of said carbon fibers comprising:
a) providing a plurality of carbon fibers;
b) treating the carbon fibers with a solution of metal catalyst salt;
c) heating the carbon fibers in hydrogen at elevated temperatures to reduce the metal salt to metal; and
d) initiating whisker growth on the surfaces of the metal deposited carbon fibers by decomposition of low molecular weight hydrocarbon gas at elevated temperature.
The present invention farther provides a process for manufacturing a filter media system comprising a ceramic composite substrate which comprises a ceramic matrix reinforced with ceramic fibers, wherein an array of ceramic fiber whiskers have been grown onto said substrate, comprising:
a) densifing the ceramic fiber with a precursor in order to deposit a ceramic matrix to produce a composite;
b) treating the composite with a solution of metal catalyst salt;
c) heating the composite in hydrogen at elevated temperatures to reduce the metal salt to metal; and
d) initiating whisker growth on at least one of the surface and the bulk of the metal coated composite by subjecting the composite to a whisker precursor.
In another embodiment, the present invention provides a process for manufacturing a filter media system comprising ceramic fibers, wherein an array of ceramic fiber whiskers have been grown onto the surfaces of said ceramic fibers comprising:
a) providing a plurality of ceramic fibers;
b) treating the ceramic fibers with a solution of metal catalyst salt;
c) heating the ceramic fibers in hydrogen at elevated temperatures to reduce the metal salt to metal; and
d) initiating whisker growth on the surfaces of the metal coated ceramic fibers by subjecting them to a whisker precursor.